This invention relates to a process for oxygenating the hypolimnion of thermally stratified water bodies such as lakes and impoundments.
A lake or impoundment may be sufficiently deep to stratify thermally during the warm season of the year as the surface layers of water rise in temperature. Due to a relatively high biological oxygen demand, the dissolved oxygen level in the hypolimnion drops to a low level unfavorable to the ecology of the body of water. Whereas the DO deficiency is caused by the high biological oxygen demand, its development and persistence is dependent upon natural thermal stratification which in effect isolates the bottom water from the source of atmospheric oxygen at the surface.
Thermal stratification may for example be typified by curve A of FIG. 1 which displays temperature variation with depth as may reasonably develop in natural course during late spring or early summer. It is seen that the surface stratum or epilimnion (0-6 ft. depth) is at a relatively uniform temperature near ambient (e.g., .about. 22.degree.-23.degree.C). Beneath this stratum is an intermediate depth of water known as the metalimnion or thermocline which exhibits a steep drop in temperature, and in FIG. 1, this drop is about 0.9.degree.C per foot depth to a temperature of about 7.degree.C at a depth of 23 feet. The hypolimnion stratum below the theromocline is at a relatively uniform cold temperature (e.g., 3.degree.-7.degree.C). In FIG. 2, stratification is represented by a warm surface layer of water 12, an intermediate thermocline (shaded stratum) 13, and hypolimnion 14.
It is usually desirable to preserve thermal stratification in lakes and reservoirs. Many forms of life such as trout can exist only at cold temperatures. The low temperature level of hypolimnetic water is often valuable and desirable when the water is used for cooling and/or domestic purposes. Destruction of stratification warms the bottom water and promotes growth and decay of aquatic plant life and accelerates biological activity. Intermixing the water strata will also raise sediments from the bottom which not only impairs clarity, but also further increases the oxygen demand of the water.
Curve B of FIG. 1 shows the dissolved oxygen profile which may typically develop as a consequence of high biological oxygen demand and thermal stratification. The DO drops below 1 ppm in the hypolimnion and is too low to support aquatic life and to avoid septicity. At least 2 ppm DDO is desirable and some forms of life require levels of 4-5 ppm.
It is known to oxygenate the hypolimnion of a thermally stratified body of water by a device resembling an inverted U-tube with the legs therof extending down into the hypolimnion. Water of low dissolved O.sub.2 content is elevated through one of the legs, aerated with an O.sub.2 containing gas (usually air), disengaged from undissolved gas bubbles, and returned down the other leg. The U-tube in some instances involves separate tubes or conduits facilitating some degree of lateral displacement of the respective ends for ingress of circulating water. In other instances, it may take the form of concentric conduits with respective ends vertically displaced.
Such devices are usually installed directly in the impoundment, either by floating the device on buoyant supports or by anchoring the device on bottom supports. The devices tend to be cumbersome and may detract from the esthetic and recreational value of the body of water. It is inconvenient and often impractical to provide service connection to the shore, e.g., power lines to drive motors for pumping and/or aeration, and a gas line to supply oxygen. To minimize off-shore power requirement, the water is usually circulated under low head, and in one known arrangement, the requirement for power is limited to the operation of an air blower to inject air into the up-flow leg of the system, thereby achieving water flow and aeration by an "air-lift" effect. In any event, the low pressure often limits both the rate of efficient O.sub.2 dissolution and the DO level of the returned water. For example, the maximum DO level obtainable with air at one atmosphere pressure in the cool water from the hypolimnion is about 10-12 mg/l. As a result, large conduits are needed for water circulation in order to minimize fluid friction and maximize volumetric flow rate. Low DO level of the returned water means that a relatively high turnover rate must be imposed on the hypolimnion in order to furnish and distribute dissolved O.sub.2 at a desired rate. High turnover tends to erode the thermocline and greatly increases the risk of completely mixing the impoundment. This limited O.sub.2 transport capability per unit mass of recirculating water is particularly disadvantageous when moderately high DO levels, e.g., 5-7 mg/l are to be maintained in the hypolimnion. Even if the recirculating water were saturated with O.sub.2 in the air, only about 5 lb. O.sub.2 could be transferred per million pounds water circulated. In some instances, it may be desirable to oxygenate the entire hypolimnion to very high levels on the order of 15-20 mg/l, and obviously such levels could not be accomplished with air unless extremely high pressures were employed.
The so-called side stream pumping concept (SSP) has also been employed to increase the dissolved oxygen level of streams and of completely mixed bodies of water. According to the SSP concept, a fraction of the body of water (stream, lake or impoundment) is withdrawn, pumped to relatively high pressure, infused with gaseous O.sub.2 and the highly oxygenated water is reinjected into the main body.
In prior art practice of SSP, the injected oxygen is not completely dissolved before re-admixture with the main body. In many instances, the amount of oxygen injected into the side stream is greater than that corresponding to the solubility limit of the water at conditions prevailing at the point of re-admixture. Hence, a two-phase mixture of gas bubbles is produced in the bulk liquid at the point of return. Such gas bubbles may be carried as a two-phase mixture through the side stream return conduit or they may be created at the point of return by throttling the side stream to supersaturated conditions.
Unfortunately, the prior art SSP concept described above is inappropriate for oxygenating the hypolimnion of a lake or impoundment. Production of gas bubbles within the hypolimnion will cause the cold liquid to up-well through the thermocline to the surface, thereby destroying stratification and completely mixing the body of water. For this reason, SSP has not been employed for oxygenating the hypolimnion. Without freedom to produce gas bubbles in the main body, the effective in situ mechanism for mass transfer is lost. Moreover, the DO-distributive effect of the gas bubbles is not available. The paradox results that water of the hypolimnion should be intermixed in order to suppress DO gradients but should not be intermixed because of the requirement to preserve thermal gradients.
Another deficiency of prior art SSP systems is the reliance upon high exit velocities at the remixing point. High exit velocities can be beneficial in applications where the body of water is already completely mixed or where complete mixing is permissible. Distribution of the highly oxygenated liquid is aided by jetting the return liquid forcefully into the main body, thereby stirring and mixing the water toward uniform DO concentration. However, for hypolimnion oxygenation, complete mixing of the water body must be avoided at all costs, and the momentum effect of return stream on the hypolimnion must be kept low.
An object of this invention is to provide an improved side stream pumping process for oxygenating the hypolimnion, which does not produce excessive gas bubbles and thereby destroy thermal stratification.
Another object of the invention is to provide an improved process which does not depend on high exit velocities at the remixing point in the hypolimnion.
Other objects and advantages of this invention will be apparent from the ensuing disclosure and appended claims.